Infrared laser catheter system

ABSTRACT

Laser energy produced by a laser operating in the mid-infrared region (approximately 2 micrometers) is delivered by an optical fiber in a catheter to a surgical site of biological tissue removal and repair. Disclosed laser sources which have an output wavelength in this region include: Holmium-doped Yttrium Aluminum Garnet (Ho:YAG), Holmium-doped Yttrium Lithium Fluoride (Ho:YLF), Holmium-doped Yttrium-Scandium-Gadolinium-Garnet (HO:YSGG), Erbium-doped YAG, Erbium-doped YLF and Thulium-doped YAG. Laser output energy is applied to a silica-based optical fiber which has been specially purified to reduce the hydroxyl-ion concentration to a low level. The catheter may be comprised of a single optical fiber or a plurality of optical fibers arranged to give overlapping output patterns for large area coverage. In a preferred application for the removal of atherosclerotic plaque, a Holmium-doped laser operating in the wavelength range of from about 1.9 to about 2.1 micrometers is preferred. For removal of such plaque by a Holmium-doped laser, it has been found that the threshold energy density should be greater than about 0.6 joules/mm 2  per pulse, and that the pulse width should be substantially less than about 83 milliseconds, and that the repetition rate should be in the range of from about 1 to about 10 Hertz.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of my application Ser. No.166,315 filed Mar. 10, 1988, now U.S. Pat. No. 4,917,084, which is acontinuation-in-part of my application Ser. No. 014,990 filed Feb. 11987, which is a continuation of my application Ser. No. 761,188 filedJul. 31, 1985, now abandoned.

FIELD OF THE INVENTION

This invention relates to laser catheters and optical fiber systems forgenerating and transmitting energy to a surgical site in a living bodyfor the purposes of tissue removal or repair

BACKGROUND OF THE INVENTION

While lasers have been used for many years for industrial purposes suchas drilling and cutting materials, it is only recently that surgeonshave begin to use lasers for surgical operations on living tissue. Tothis end, laser energy has been used to repair retinal tissue and tocauterize blood vessels in the stomach and colon.

In many surgical situations, it is desirable to transmit laser energydown an optical fiber to the surgical location. If this can be done, theoptical fiber can be included in a catheter which can be inserted intothe body through a small opening, thus reducing the surgical traumaassociated with the operation. In addition, the catheter can often bemaneuvered to surgical sites which are so restricted that conventionalscalpel surgery is difficult, if not impossible. For example, laserenergy can be used to remove atherosclerotic plaque from the walls ofthe vasculature and to repair defects in small-diameter artery walls.

A problem has been encountered with laser surgery in that prior artlasers which have been used for industrial purposes often havecharacteristics which are not well suited to percutaneous laser surgery.For example, a laser which is conventionally used for scientificpurposes is an excimer laser which is a gas laser that operates with agas mixture such as Argon-Fluorine, Krypton-Fluorine or Xenon-Fluorine.Another common industrial laser is the carbon dioxide or CO₂ laser.

Both the excimer laser and the CO₂ laser have been used for surgicalpurposes with varying results. One problem with excimer lasers is thatthey produce output energy having a wavelength in the range 0.2-0.5micrometers. Blood hemoglobin and proteins have a relatively highabsorption of energy in this wavelength range and, thus, the output beamof an excimer laser has a very short absorption length in thesematerials (the absorption length is the distance in the materials overwhich the laser beam can travel before most of the energy is absorbed).Conseguently, the surgical site in which these lasers are to be usedmust be cleared of blood prior to the operation, otherwise most of thelaser energy will be absorbed by intervening blood before it reaches thesurgical area. While the removal of blood is possible if surgery isperformed on an open area it is often difficult if surgery is to beperformed via a catheter located in an artery or vein.

An additional problem with excimer lasers is that the output energypulse developed by the laser is very short, typically about tennanoseconds. In order to develop reasonable average power, pulses withextremely high peak power must be used. When an attempt is made tochannel such a high peak power output into an optical fiber, the highpeak power destroys the fiber. Thus, excimer lasers have a practicalpower limit which is relatively low. Conseguently, when these lasers areused for biological tissue removal, the operation is slow and timeconsuming.

The CO₂ generates output energy with a wavelength on the order of 10micrometers. At this wavelength, the absorption of blood hemoglobin isnegligible but the absorption by water and tissue is relatively high.Scattering at this wavelength is also very low. Although thesecharacteristics of the CO₂ laser are favorable for surgicalapplications, the CO₂ laser suffers from the same drawback as excimerlasers in that the absorption length is relatively short due to the highabsorption of the laser energy in water. Thus, the surgical area must becleared of blood prior to the operation.

Unfortunately, the CO₂ laser also suffers from a serious additionalproblem. Due to the long wavelength, the output energy from the carbondioxide laser cannot be presently transmitted down any optical fiberswhich are suitable for use in percutaneous surgery (present fibers whichcan transmit energy from a CO₂ laser are either composed of toxicmaterials, are soluble in water or are not readily bendable, or possessa combination of the previous problems) and, thus, the laser is onlysuitable for operations in which the laser energy can be either applieddirectly to the surgical area or applied by means of an optical systemcomprised of prisms or mirrors.

Accordingly, it is an object of the present invention to provide a lasercatheter system which uses laser energy of a wavelength that is stronglyabsorbed in water, in bodily tissues and atherosclerotic plaque.

It is another object of the present invention to provide a lasercatheter system which is capable of providing laser energy that can betransmitted through existing silica-based optical fibers.

It is a further object of the present invention to provide a lasercatheter system in which optical scattering is minimized and which has amedium-length absorption length to confine the energy to the area ofinterest.

It is yet another object of the present invention to provide an opticalcatheter system with a laser that can be operated on either a pulsedmode or a continuous wave mode.

It is still another object of the present invention to provide a lasercatheter system which can be used for biological material removal andbiological material repair.

It is still another further object of the present invention to provide alaser catheter system which can be used for removal of atheroscleroticplaque within a living body.

SUMMARY OF THE INVENTION

The foregoing objects are achieved and the foregoing problems are solvedin one illustrative embodiment of the invention in which a lasercatheter system employs a laser source operating in the wavelengthregion of 1.4-2.2 micrometers. Illustrative laser sources operating thisregion are Holmium-doped YAG, Holmium-doped YLF, Holmium-doped YSGG,Erbium-doped YAG, Erbium-doped YLF and Thulium-doped YAG lasers.

In the inventive laser system, the above-noted lasers are used with aspecially-treated silica fiber that has been purified to reduce theconcentration of hydroxyl (OH--) ions.

For biological tissue removal, the laser source may be operated in apulsed mode with a relatively long pulse of approximately 0.2-5milliseconds at an energy level of approximately 1-2 joules per pulse,for a spot size of the order of 1.5 millimeters in diameter. With thistime duration and energy level, the peak power of the laser pulse isapproximately 1 kilowatt. This amount of power can easily be toleratedby the silica fiber, but is sufficient for rapid material removal. Witha repetition rate in the range of 1-10 hertz, the average powerdelivered to a surgical site by such a laser will be under 10 watts.

In particular, for removal of atherosclerotic plaque from a living body,particularly satisfactory results are obtained using a Holmium-dopedlaser source operating in a pulsed mode in a wavelength range of fromabout 1.90 to about 2.10 micrometers, and at a threshold energy densityof at least about 0.6 joules/mm². The pulse width used should besubstantially less than a thermal time constant for plaque, orsubstantially less than about 83 milliseconds. The repetition ratetypically is about 2 Hertz.

Alternatively, for biological tissue repair, the laser source can beoperated in a low power continuous wave mode to repair, by coagulation,of tissue by a process similar to "spot welding".

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, advantages, and features of this invention will be moreclearly appreciated from the following detailed description when takenin conjunction with the accompanying drawings in which:

FIG. 1 shows a sketch of absorption of electromagnetic energy versuswavelength and electromagnetic energy scattering versus wavelength.

FIG. 2 shows an absorption versus wavelength plot for atheroscleroticplaque obtained in a carotid endarterectomy with the region of interestfor the inventive laser sources (1.4-2.2 micrometers) outlined.

FIG. 3 of the drawing is a schematic diagram of a typical solid statelaser construction used in the inventive laser sources.

FIG. 4 of the drawing is a plot of laser output intensity versus timefor a typical pulse shape developed by a laser shown in FIG. 3 when usedfor tissue removal.

FIG. 5 is a schematic diagram of a laser catheter which employs a singleoptical fiber for transmitting laser energy to a surgical location.

FIG. 6 of the drawing is an enlarged cross-section of the probe tip thesingle fiber catheter shown in FIG. 5.

FIG. 7 is an exploded view of a portion of the enlarged cross-section ofthe probe tip shown in FIG. 6.

FIG. 8 is a schematic diagram of a wire-guided catheter which employsfour optical fibers to increase the area which can be irradiated withthe laser light.

FIG. 9 of the drawing is an enlarged cross-sectional view of the probetip of the catheter shown in FIG. 7 showing the four optical fibers.

FIG. 10 is an end view of the probe tip of the catheter in the direction10--10 of FIG. 9.

FIGS. 11, 11A, 11B, and 11C are schematic diagrams of the beam patternproducted by the four-fiber catheter at the surgical location.

FIG. 12 is a schematic diagram illustrating the apparatus used to obtainin vitro data relating to the removal of atherosclerotic plaque.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The absorption and scattering characteristics versus output wavelengthof a plurality of known laser systems are shown in FIG. 1. FIG. 1 has alogarithmic scale representing the absorption coefficient in units ofcm⁻¹ along the vertical axis and the incident energy wavelength inmicrometers along the horizontal axis.

From FIG. 1, it can be seen that excimer laser systems which utilizeconventional gas mixtures, such as Argon-Fluorine, Krypton-Fluorine andXenon-Fluorine, and Argon gas lasers produce output energy which fallsin the 0.2-0.5 micrometer wavelength region. In this region, theabsorption of blood hemoglobin and proteins is very high. Conseguently,the absorption length is very short (about 5-10 microns) and virtuallyno blood can be present between the fiber end and the surgical siteduring the operation. Thus, it is necessary to remove blood from thesurgical area when these lasers are used for surgical purposes.

In addition, for lasers such as Argon, the absorption of water reaches aminimum at 0.5 micrometers so that it is necessary to use a higher powerlaser than is desirable to achieve sufficient power in the surgical areafor material cutting and removal. Also, due to the low absorption of thelaser output in water and hemoglobin, the absorption length is very long(approximately 1 mm). In addition, scattering for these lasers isrelatively high, causing difficulty in controlling the laser energy anda danger of tissue damage outside the surgical area due to scattering ofthe laser energy.

At the other end of the wavelength spectrum shown in FIG. 1 are carbonmonoxide and carbon dioxide lasers producing outputs at 5 and 10micrometers, respectively. At these wavelengths scattering is negligibleand absorption by water and tissue is relatively high and thus bothlasers have good surgical properties. Unfortunately, due to the highabsorption of water, the absorption length is relatively short (about 20microns). Further, silica-based optical fibers in present use which aresuitable for percutaneous surgical use have a practical "cutoff" intransmission which occurs approximately at 2.3 micrometers, and, thus,the output energy from carbon monoxide and carbon dioxide lasers cannotbe transmitted through such an optical fiber.

In accordance with the invention, laser sources of interest are thosethat lie in the wavelength range of approximately 1.4-2.15 micrometers.As shown in FIG. 1, in this range, the energy absorption of water isrelatively high whereas optical scattering is relatively low.Illustrative lasers which are useful with the present invention compriseErbium-doped Yttrium Aluminum Garnet (YAG) with a wavelength of 1.55micrometers, Erbium-doped Yttrium Lithium Fluoride (YLF) with awavelength of 1.73 micrometers, Thulium-doped YAG with a wavelength of1.88 micrometers, Holmium-doped YLF with a wavelength of 2.06micrometers, Holmium-doped YAG at a wavelength of 2.1 micrometers, andHolmium-doped Yttrium-Scandium-Gadolinium-Garnet (YSGG) at a wavelengthof 2.088 micrometers. The absorption of the laser energy produced bylasers in this latter group by water is moderately high and,conseguently, the absorption by biological tissues of such energy willalso be relatively high. However, the absorption by water is not as highas the absorption of CO and CO₂ laser energy. Thus, the absorptionlength will be longer for the lasers operating in the 1.4-2.2 micrometerrange than for CO₂ lasers. Typically, the absorption length in the bodyfor lasers operating in the 1.4-2.2 micrometer range is about 200microns. Therefore, it is still possible to operate satisfactorily evenwith 10-30 microns of blood between the fiber end and the surgical site.

Of particular interest is the absorption of the laser energy byatherosclerotic plaque, since an important use of laser catheter systemsis angioplasty, particularly the clearing of blocked arteries. FIG. 2 isa plot of the absorption by plaque of electromagnetic energy versuswavelength for energy in the wavelength range of 0.2-2.2 micrometers. Asshown in FIG. 2, the absorption by plaque of electromagnetic energyreaches a minimum in the 0.8-1 micrometer wavelength range and generallyincreases with increasing wavelength in the wavelength region of 1-2.2micrometers. In the wavelength range from 1.4-2.2 micrometers, thewavelength range produced by laser in the above-mentioned group, theabsorption by plaque is at a relatively high value.

A schematic diagram of a typical solid-state laser construction is shownin FIG. 3. The laser assembly consists of a laser crystal 1 and anexcitation device such as a flashlamp 3. Typically, for the crystalcompositions disclosed above, the laser crystal must be cooled tocryogenic temperatures to provide low laser-threshhold operation.Cryogenic cooling is typically provided by enclosing crystal 1 in aquartz or fused-silica jacket 4 through which liquid nitrogen iscirculated. Liquid nitrogen enters jacket 4 by means of an inlet pipe 5and leaves by means of an outlet pipe 6. The laser cavity is formed by ahigh-reflectivity concave mirror 10 and a partial reflector 12.

Generally, the crystal is excited by optical pumping which is, in turn,accomplished by irradiating the crystal with light from a flashlamp 3. Aflashlamp which is typically used with the inventive laser compositionsis a high-pressure Xenon flashlamp. Lamp 3 may also be surrounded by aquartz flow tube (not shown) through which coolant is pumped.

Crystal 1 and flashlamp 3 are enclosed in a reflector 2 whichconcentrates the flashlamp energy into the laser crystal. To maximizeenergy transfer from lamp 3 to crystal 1, the inner surface of reflector2 is coated with a material chosen to have high-reflectivity at thepumping wavelength of the laser crystal--illustratively, aluminum orsilver. In order to provide thermal insulation to prevent condensationon the system optics, it may be necessary to evacuate the interior ofreflector 2 or to provide a vacuum jacket around crystal 1.

The construction of cryogenic solid-state lasers is conventional anddescribed in a variety of sources; accordingly such construction willnot be discussed further in detail herein. A more complete discussion ofconstruction details of a typical cryogenic laser is set forth in anarticle entitled "TEM_(oo) Mode Ho:YLF laser", N. P. Barnes, D. J.Gettemy, N. J. Levinos and J. E. Griggs, Society of Photo OpticalInstrumentation Engineers, Volume 190--LASL Conference on Optics 1979,pp 297-304.

FIG. 4 of the drawing is a plot of the illustrative pulse shapedeveloped by a laser in the preferred group when used in the "materialremoval" mode. FIG. 4 shows light intensity along the vertical axisincreasing in the downward direction versus time increasing towards theright. Although, as shown in FIG. 4, the laser source has been adjustedto produce an output pulse of relatively long time duration, most of theoutput energy is released within approximately 1 millisecond of thebeginning of the pulse. It should also be noted, as illustrated in FIG.4, that lasers in the preferred laser group exhibit a "spiking"phenomenon caused by internal relaxation-oscillations in the lasercrystal. The spiking behavior causes local increases in laser intensitywhich have a large magnitude, but a very short time duration. Similar"spiking" behavior has been found advantageous when lasers are used todrill metals and other materials for industrial purposes and it isbelieved that such "spiking" behavior enhances the laser usefulness forbiological material removal.

FIG. 5 is a schematic diagram of a laser/catheter system employing asolid state laser of the type shown in detail in FIG. 3. Morespecifically, the infrared output energy of laser 21 is focused by aconventional focusing lens onto the end of the optical fiber which isheld in a conventional fiber optic connector 24. Fiber optic connector24 is, in turn, connected to a tube 27 which houses a single opticalfiber. Tube 27 is connected to a conventional two-lumen catheter 30 bymeans of a bifurcation fitting 28.

Illustratively, catheter 30 has two lumens passing axially therethroughto its distal end 34 so that an optical fiber can pass through one lumenand transmit laser energy from fiber optic connector 24 to lens tip 34.As previously mentioned, the optical fiber which passes through thecatheter is specially purified to reduce the hydroxyl ion concentrationto a low level, thus preventing the laser energy which is transmitteddown the fiber from being highly absorbed in the fiber material. A fiberwhich is suitable for use with the illustrative embodiment is afused-silica optical fiber part no. 822W manufactured by the SpectranCorporation located in Sturbridge, Mass.

Advantageously, the mirrors and lenses (10, 12 and 22) which are used toform the IR laser cavity and focus the output energy beam are generallyonly reflective to energy with a wavelength falling within a narrowwavelength band and transparent to energy at other wavelengths.Conseguently, the mirrors and lenses are transparent to visible light.An aiming laser 20 (for example, a conventional helium-neon laser) whichgenerates visible light may be placed in series with IR laser 21 togenerate a visible light beam. This light beam may be used to alignmirrors 10 and 12 and to adjust focussing lens 22 so that the opticalfiber system can be aligned prior to performing surgery.

Also, the optical fibers used to transmit the IR energy from laser 21 tothe surgical area can also be used to transmit the visible light fromthe aiming laser 20 to the surgical area. Thus, when the inventivesystem is used in performing surgery where the surgical area is visibleto the surgeon, the light produced by laser 20 passes through theoptical fiber in catheter 30 and can be used to aim the probe tip beforelaser 21 is turned on to perform the actual operation.

The second lumen in catheter 30 is provided for transmission of aflushing fluid or to apply suction to the probe lens tip area to clearthe area of blood during surgery. This latter lumen is connected throughbifurcation fitting 28 to a second tube 29. Tube 29 may illustrativelybe terminated by a standard Luer-Lok fitting 26 which allows connectionof the catheter to injectors and standard flow fittings. Solutionsinjected into the catheter through tube 29 pass through the lumen incatheter 30 and exit at the distal end via a small orifice 32.

Probe tip 34 consists of a lens arrangement which forms the laser energyinto a beam 36 which is used to perform the surgical operations. Anenlarged view of the probe tip is shown in FIGS. 6 and 7.

To ensure that the distal end of optical fiber 18 is spaced and orientedin a precise position with respect to the end of the probe, fiber 18 ismounted in a high-precision holder 58 which has a reduced diameter end64 that forms a shoulder 68. Shoulder 68, as will hereinafter bedescribed, is used to hold the probe tip assembly together. Holder 58has a precision-formed axial bore made up of two sections, including alarge-diameter section 60 and a narrow-diameter section 63. Holder 58may be made of glass, ceramic or other material capable of being formedto specified dimensions with precise tolerances.

In order to attach holder 58 to the end of fiber 18, the fiber is firstprepared as shown in FIG. 7. More particularly, prior to insertion offiber 18 into holder 58, a portion of buffer sheath 61 is removed,exposing a length of optically-conductive core 65. Care is taken whenstripping buffer sheath 61 from the fiber not to damage the layer ofreflective cladding 67 located on the surface of core 65. Afterstripping, fiber 18 is inserted into holder 58 so that core 65 extendsinto the small-diameter bore 63 and sheath 61 extends into thelarge-diameter bore 60. After fiber 18 has been inserted into holder 58,it may be fastened by epoxy cement to permanently affix the components.To complete the assembly, the end of fiber 18 which protrudes beyondsurface 62 of holder 58 may be finished flush with the surface bygrinding the assembly or by carefully cleaving the fiber.

Referring to FIG. 6, holder 58 (with fiber 18 fastened inside) ismounted within a glass tube 51 to shield the assembly. The frontsurface, 62, of holder 58 is spaced from the inner surface 142 of planarlens 144, which may be comprised of glass or sapphire, by means of aspacing ring 154. Ring 154 may illustratively be made of radiopaquematerial so that the catheter tip can be located inside the patient bymeans of a fluoroscope.

Glass tubing 51 is bent over shoulder 68 of holder 58 to form aconstricted end, 65, which holds the probe tip assembly together. Afiller, 66, which may be made of a plastic such as TEFLON (trademark ofthe DuPont corporation for polytetrafluoroethylene) fills the annularspace between catheter body 30 and end 65 of glass tube 51. The outerdiameter of the entire assembly from catheter body 30 to glass tube 51is substantially the same, providing a smooth, uniform surface along theentire length of the catheter as indicated in FIG. 6.

FIG. 8 shows a schematic diagram of a wire-guided, four-fiber catheterfor use with the present invention. The laser system is set up aspreviously described with the infrared laser 21 constructed inaccordance with the above disclosure. A visible helium-neon aiming laser20 may also be used in line with laser 21 for aiming purposes asdiscussed with the single fiber catheter. The output of the infraredlaser 21 is directed towards a set of four mirrors 160-168 arranged at a45° angle with respect to the axis of beam 14.

The first mirror, 160, has a 25% reflective surface and directsapproximately 1/4 of the energy to focusing lens 70. The second mirrorof the set, 162, is a 33% reflector which directs 1/4 of the totalenergy to focusing lens 72. Mirror 164 is a 50% reflector which directs1/4 of the total laser output to focusing lens 74. The last mirror inthe set, mirror 168, is a 100% reflector which directs the remain 1/4 ofthe total energy to focusing lens 78. Mirrors 160-168 and lenses 70-78are conventional devices.

Focusing lenses 70-78 focus the output energy from IR laser 21 onto fourfiber optic connectors 80-88. Connectors 80-88 are connected,respectively, to tubes 90-96 which are all connected, via a branchconnector 102, to catheter 104. Each of tubes 90-96 contains a singleoptical fiber which transmits 1/4 of the total laser output energythrough the catheter body to the catheter tip 108. An additional tube 98is provided which is connected to branch fitting 102 and to aconventional Luer Lok connector, 100. This latter tube is connected to acentral lumen in catheter body 104 through which flushing solutions maybe injected or through which a guide wire may be inserted through thecatheter for purposes of guiding the catheter to the surgical area.

At catheter tip 108, the four optical fibers which pass through thecatheter are arranged symmetrically so that the beams 110 overlap toilluminate a larger area. Tip 108 also has a hole on the center thereof,through which guidewire 112 can protrude to direct the catheter to theproper location.

FIGS. 9 and 10 show detailed views of the illustrative four-fibercatheter tip. The four optical fibers 42 and the inner shaft 40 whichholds the fibers, are held in a fiber holder 50 which is preferablyformed from a radiopaque material such as stainless steel or platinum.Fiber holder 50 is cylindrical and is provided with a central aperture,54, which communicates with a lumen 34 of approximately the same sizethat passes through the center of the catheter body 104. Fiber holder 50is provided with a plurality of longitudinally extending holes 56 whichextend through the wall of holder 50 and receive, in a snug fit, thedistal ends of the fiber cores 42. The distal face 158 of the combinedfiber cores 42 and holder 50 is polished flat to butt flush againstoptically transparent cap 52.

Cap 52 is cylindrical and has the same outer diameter as catheter body104 so that the two pieces define a smooth and continuous diameter. Cap52 may be formed of a transparent substance such as pyrex glass orsapphire and has an enlarged bore 262 extending in from its proximalend. Bore 262 terminates at its end to form internal shoulder 260. Asmaller diameter central aperture, 264, is formed in the distal end ofcap 52 which aperture may have the same diameter as aperture 54 in fiberholder 50 and lumen 34 in catheter body 104 to provide a smooth andcontinuous lumen which opens at the distal tip of the catheter. However,the aperture 264 in tip 52 may also be somewhat narrower than aperture14 and lumen 34 as long as sufficient clearance is provided toaccommodate a guidewire without adversely interfering with fluid flowand pressure measurements.

Cap 52 is secured by an epoxy adhesive (placed on the inner surface ofbore 262 to the fiber holder also to the portion of the inner shaft 40and fibers 42 which are disposed within the proximal end of the cap 52.The distal end of the catheter body 104 is heat shrunk around the innershaft 40 and fibers 42 to provide a smooth transition from cap 52 tocatheter body 104.

More complete construction details of a four fiber catheter suitable foruse with the illustrative embodiment are given in co-pending U.S. patentapplication entitled "Wire Guided Laser Catheter", filed on May 22, 1985by Stephen J. Herman, Laurence A. Roth, Edward L. Sinofsky and DouglasW. Dickinson, Jr. now U.S. Pat. No. 4,850,351.

FIG. 11 illustrates the output beam pattern developed by a four-fibercatheter, such as that described above, in which the four fibers arearranged in two diametrically-opposed pairs. The beam pattern from eachof the four fiber ends is defined by a cone formed by the ray lines 70in FIG. 11. The beam from each individual fiber 42 is emitted from thedistal face of the fiber 42 and enters the distal segment 72 of cap 52through the face defining the shoulder 260. The beam from each fiber isdivergent and, the illustrative embodiment, may have a half-angle in therange of 6°-16°, depending on the numerical aperture of the fibers whichare used to construct the catheter.

The diverging beam from each of the fibers 42 exits from the distalemission face 74 at the end of cap 52. FIGS. 11A, 11B and 11C illustratethe overall beam pattern (in cross-section) which is formed by theoutput of the four fibers as seen along image planes 11A, 11B and 11C inFIG. 11. At plane 11A, which is located at the emission face 74 of cap52, the four beams in the illustrative embodiment are still separate. Atplane 11B, the diverging beams have spread further and have begun tooverlap. At the plane indicated as 11C, the beams have overlapped anddefine an envelop 73 having an outer diameter which is slightly greaterthan the outer diameter of the catheter body 104. Preferably, at plane11C, beams 70 will have overlapped to merge and cover a continuouspattern. Illustratively, such a merger will have occurred within adistance from the distal face 74 of tip 52 which is approximately equalto the outer diameter of catheter 104 (a typical diameter is 1.5millimeters).

A preferred application of the previously described laser cathetersystem of this invention is the removal of atherosclerotic plaque. Thelaser must be operated in a pulsed mode to remove such plaque. Acontinuous wave laser is unsuitable for plaque removal, since acontinuous wave laser transmits insufficient energy to the surgical siteat a time to vaporize the tissue before the energy is dissipated bythermal diffusion. As a consequence, typically, carbon deposits areformed in the affected area, and it is difficult to cleanly andaccurately remove plaque from artery walls using a continuous wavelaser.

Although any of the lasers previously mentioned as operating in thepreferred range of wavelengths would be suitable for the removal ofatherosclerotic plaque, it has been found that Holmium-doped lasersoperating at a wavelength in the range of from about 1.90 to 2.10micrometers are particularly suited for such surgical applications. Inthe first place, Holmium-doped lasers operate in the preferred range ofwavelengths of 1.4-2.2 micrometers, and they can be operated in a pulsedmode. Secondly, Holmium-doped lasers are generally very efficient,generating 10 watts of average power for every kilowatt of electricalpower supplied to the lamp. Because of these power requirements, whichare lower than for most conventional lasers, no open loop cooling isrequired. Furthermore, such lasers can be operated with a conventional110 volt power source. As a consequence of the foregoing, Holmium-dopedlaser sources often are portable, and can be operated either in ahospital environment, or in a doctor's office, or even in the home. Alaser source which is preferred is a Holmium-dopedYttrium-Scandium-Gadolinium-Garnet (YSGG) laser operating at awavelength of 2.088 micrometers. Such a laser can remove any tissuecontaining water.

The pulse width of any laser which is used for plaque removal should besubstantially less than the thermal time constant for that particulartissue. Typically, the pulse width should be roughly an order ofmagnitude less than the thermal time constant. For wavelengths in therange of 1.4-2.2 micrometers, thermal diffusion, which is a function ofabsorption, is reduced to an insignificant factor when such pulse widthsare used. For a given power or energy level delivered per pulse, pulseslonger than the thermal time constant will be generally incapable ofprecisely removing tissue, because the energy delivered thereby will bedissipated through thermal diffusion.

For all tissue, the thermal time constant is equal to the absorptionlength of that particular tissue type squared over four times itsthermal diffusivity. For most tissue, the thermal diffusivity is equalto about 1×10⁻³ cm² /sec, while for atherosclerotic plaque, the thermaldiffusivity is equal to about 1.2×10⁻³ cm² /sec. The absorption lengthfor most types of plaque is about 2×10⁻² cm. Thus for plaque, thethermal time constant is approximately 8.3×10⁻² seconds. Therefore,typically, the pulse width should be of the order of about 8.3milliseconds or less.

The repetition rate is not critical, and typically is in the range of1-10 Hertz. The preferred rate, for use with a Holmium-doped laser, isabout 2 Hertz.

The energy density delivered is defined as the energy per pulse dividedby the spot size. The spot size is the area of the spot upon which thelaser energy is incident. It has been predicted theoretically andverified experimentally that for plaque removal, the energy density ofeach pulse should exceed some threshold value. If the energy density isless than this threshold value, the energy will be dissipated by thermaldiffusion before the tissue is vaporized, and no vaporization willoccur. It has been found that most energy densities above the thresholdvalue can be used, up to the operational limit of the laser and theassociated fiber optic system.

The threshold value of the energy density for plaque will vary,depending upon the particular tissue sample involved. Generally, tissuesamples with more calcification or less water content will vaporize at ahigher threshold value, while tissue samples with less calcification, ora higher water content, will vaporize at a lower threshold value.

A thermal model was developed for laser pulses whose pulse widths aresmall compared to the thermal time constant of the material with whichthe laser is to be used. Using this thermal model, one can developpredicted threshold energy densities per pulse for lasers operating atdifferent wavelengths, as well as the extent of thermal damage or theextent of the denatured rim.

In using this thermal model, the energy profile in the tissue can bedescribed as a function with separable axial and radial components. Theaxial component can be described by the attentuation coefficientaccording to Beer's law and the radial component can be described by alinearly expanding Gaussian distribution. The axial and radialcomponents can be defined mathematically as follows:

    J(r,z)=J(r)·J(z)

where

(z)=J_(o) e⁻βz

and J(r)=e^(-2r) ² ^(/x) ²,

where the function J(r,z) is the spatial energy distribution, z is theaxial distance into the tissue measured from the tissue-air ortissue-water interface, α is the combined absorption-scatteringattenuation coefficient, r is the radial distance measured from theoptical axis, and x is the 1/e² beam width of the Gaussian-like radiallight distribution.

The amount of energy, Δ J, deposited into a volume element is equal tothe energy profile divided by the volume as a function of the radialdistance measured from the optical axis or ##EQU1##

Besides the attenuation coefficient, the next most important parameteris a description of the 1/e² radius, x. Beam spreading occurs due tomultiple scattering. Measurements have shown that the beam spreading islinear in almost all cases, although the slope varies with differenttissue samples. Because absorption dominates over scattering inmid-infrared wavelengths, spreading in this range should be quite small.Therefore, it is assumed that the radius expands with the samedivergence as the incident beam.

The threshold energy density is the sum of the energy required to raisethe tissue temperature to 150° C. from the ambient temperature of about37° C. plus the energy required to overcome the heat of vaporization(2260 J/g). Thus, ##EQU2## where p is the tissue density (1.2 g/cm³), cis the specific heat (3.6 J/g° C.), and L_(v) is the heat ofvaporization (2260 J/g). It can be seen that the total energy densityrequired to vaporize a volume element is 3200 J/cm³. When the depositedenergy density is between 488 and 3200 J/cm³, the tissue temperatureremains at 150° C.

For purposes of comparison, the vaporization threshold is defined asbeing reached if enough energy is supplied to vaporize tissue out to theedge of a 1 millimeter hole. This definition means that both the centralpeak and the wings of the Gaussian energy distribution are sufficient tovaporize tissue. The thermal zone of damage is defined as the region inwhich the temperature has reached above 60° C., and can vary in theaxial and radial directions. Energy densities in terms of joules/mm² fordifferent wavelengths can be determined based on the assumption that thedepth of the hole is equal to the absorption length of the radiation atthat wavelength.

Based upon the foregoing analysis, the following Table I sets forth thepredicted threshold energy densities in joules per mm² and the predictedsizes of the zones of axial and radial thermal damage in micrometers forthe lasers previously indicated to operate within the preferred range ofwavelengths. All of these predicted energy densities are foratherosclerotic plaque having a high water content and little or nocalcification.

                  TABLE I                                                         ______________________________________                                        Wave-       Absorption                                                                              Threshold  Thermal                                      Length      Coeff     Energy Den-                                                                              Damage (μm)                               Laser   (μm) (cm.sup.-1)                                                                             sity (J/mm.sup.2)                                                                      Axial Radial                               ______________________________________                                        Ho:YAG  2.1     50        .76      500   500                                  Ho:YSGG 2.088   50        .76      500   500                                  Er:YAG  1.55    2700      .0095     12    12                                  Ho:YLF  2.06    50        .76      100   300                                  Tm:YAG  1.88    50        .76      100   300                                  Fr:YLF  1.73    15        5.7      500   300                                  ______________________________________                                    

Energy densities required to vaporize atherosclerotic plaque using aHolmium-doped laser also have been determined empirically from theexperimental data. These threshold energy densities were determinedusing an apparatus 120 shown schematically in FIG. 12. Apparatus 120includes a laser source 122, focusing lens and fiber optic connector124, fiber optic 126, and a tank 128 containing the tissue sample 130.Laser source 122 and connector and associated lens 124 are bothconventional, as previously described with respect to FIGS. 3-11. Fiberoptic 126 may contain a single optical fiber, or it may contain a bundleof optical fibers. Typically, in conducting these experiments, a singlefiber was used.

Tank 128 contains a saline solution in which the tissue sample 130 isimmersed. Each optical fiber included within fiber optic 126 typicallyis a 100 micron fiber. Also immersed in the saline solution in tank 128and covering tissue sample 130 is a sapphire plate 132. The end of fiberoptic 126 is placed in direct contact with the upper surface of plate132, while the low surface of plate 132 is in direct contact with tissuesample. In conducting these experiments, typically the sapphire plate132 had a thicken 1.52 millimeters. The sapphire plate 132 is used topermit a controlled spreading of the beam of emitted from the end offiber optic 126 to desired spot size on the tissue sample.

Tissue sample 130 us each of these experiments comprises a of a fattyfibrous plaque 134 disposed on a section of an aorta wall 136. Twodifferent samples were used. Experiments 1-9 were per on Sample No. 2while experiments 10-13 were as Sample No. 1. Sample No. 1 exhibitedcalcification and lower water content than Sample No. 2. Sample No. 2exhibited virtually no calcification and a high water content. Prior touse, the tissue samples had been removed from the body, and had beencarefully frozen and stored at a temperature of -70° C. The tissuesamples were later thawed for use in these experiments over a lengthyperiod of time to avoid damage to the tissue and to avoid destruction ofthe cells during the freezing and thawing processes.

In each experiment, a Holmium-doped YSGG laser was operated in a pulsedmode at a wavelength of 2.088 micrometers. The pulse width used was 400microseconds, and two pulses were emitted for each test with a spacingof one half second, or at a rate of 2 Hertz. The spot sizes and energiesper pulse were varied to determine the threshold energy density. Theresults of these experiments are set forth below in Table II.

                                      TABLE II                                    __________________________________________________________________________    Exper-                                                                            Energy/ Delivery                                                                             Fluence,                                                                             Ablation      Thermal,                              iment                                                                             Pulse, (Joules)                                                                       Area, (mm.sup.2)                                                                     Joules/mm.sup.2)                                                                     Area, (mm.sup.2)                                                                     OD, (mm)                                                                             Damage (mm)                           __________________________________________________________________________    1   122 × 10.sup.-3                                                                 4.012 × 10.sup.-1                                                              0.304  N/A    N/A    N/A                                   2   222.5 × 10.sup.-3                                                               4.012 × 10.sup.-1                                                              0.555  N/A    N/A    N/A                                   3   310 × 10.sup.-3                                                                 4.012 × 10.sup.-1                                                              0.773  3.848 × 10.sup.-1                                                              100 × 10.sup.-6                                                                50 × 10.sup.-6                  4   816.2 × 10.sup.-3                                                               3.14 × 10.sup.-2                                                               25.98  6.158 × 10.sup.-2                                                              280 × 10.sup.-6                                                                240 × 10.sup.-6                 5   475 × 10.sup.-3                                                                 3.14 × 10.sup.-2                                                               15.13  4.337 × 10.sup.-2                                                              235 × 10.sup.-6                                                                240 × 10.sup.-6                 6   268.7 × 10.sup.-3                                                               3.14 × 10.sup.-2                                                               8.55   5.41 × 10.sup.-3                                                               262.5 × 10.sup.-6                                                              100 × 10.sup.-6                 7   875 × 10.sup.-3                                                                 5.3 × 10.sup.-1                                                                1.65   5.03 × 10.sup.-1                                                               800 × 10.sup.-6                                                                35 × 10.sup.-6                  8   760 × 10.sup.-3                                                                 5.3 × 10.sup.-1                                                                1.434  6.36 × 10.sup.-1                                                               900 × 10.sup.-6                                                                35 × 10.sup.-6                  9   655 × 10.sup.-3                                                                 3.14 × 10.sup.-2                                                               20.85  1.018 × 10.sup.-1                                                              360 × 10.sup.-6                                                                200 × 10.sup.-6                 10  185 × 10.sup.-3                                                                 3.14 × 10.sup.-2                                                               5.89   6.157 × 10.sup.-2                                                              280 × 10.sup.-6                                                                35 × 10.sup.-6                  11  139 × 10.sup.-3                                                                 3.14 × 10.sup.-2                                                               4.43   4.869 × 10.sup.-2                                                              249 × 10.sup.-6                                                                10 × 10.sup.-6                  12  271 × 10.sup.-3                                                                 3.14 × 10.sup.-2                                                               8.63   4.869 × 10.sup.-2                                                              249 × 10.sup.-6                                                                25 × 10.sup.-6                  13  74.5 ×  10.sup.-3                                                               3.14 × 10.sup.-2                                                               2.37   N/A    N/A    N/A                                   __________________________________________________________________________

The value for the energy per pulse in Table II was measured in aconventional manner. The delivery area in square millimeters wascalculated from the known, controlled spreading of the light beam as itpassed from fiber optic 126 through sapphire plate 132. The fluence, injoules per square millimeter was calculated by dividing the energy perpulse by the delivery area. The ablation area is the area of the holeformed in the tissue sample. The extent of thermal damage is thedistance beyond the rim of the hole where thermal damage was observed,while the 0D is the outside diameter of the hole formed. The ablationarea, 0D and extent of thermal damage were all measured microscopicallyfrom histological slides prepared from the specimens using conventionalhistological techniques.

The immersion of a tissue sample in a saline solution, as shown in FIG.12, closely models the conditions inside the body. Although all of theseexperiments were conducted in vitro, as shown, one could expect nearlyidentical results in vivo.

It can be seen from Table II that, for fatty fibrous plaque, having ahigh water content and virtually no calcification, the threshold energydensity per pulse is at least about 0.6 J/mm² for a Holmium-doped YSGGlaser operating at a wavelength of 2.088 micrometers. It is expectedthat similar experiments using a Ho:YAG laser and a Ho:YLF laser wouldshow that the energy threshold for such lasers is also at least about0.6 J/mm² per pulse at their respective wavelengths of operation, basedupon the fact that identical threshold energy densities are predicted bythe theoretical model, as shown in Table I.

For the other lasers described herein as being suitable, thetheoretically predicted threshold energy densities per pulse forvaporization of fatty fibrous plaque having virtually no calcificationand a high water content are as follows as found in Table I: Erbium YAG,9.5 mJ/mm² ; Erbium YLF, 5.7 J/mm² ; and Thulmium YAG, 0.76 J/mm². Theprecise threshold energy density depends upon the wavelength, and thedegree of calcification and water content of the tissue sample. Asindicated, the greater the calcification and the less the water content,the higher the threshold energy density required for vaporization of thetissue.

The data set forth in Tables I and II also illustrate additionaladvantages of using a Holmium-doped laser within the wavelength rangesof 1.90 micrometers to 2.1 micrometers. It can be seen that the zone ofthermal damage is minimal, far less than one would expect from the priorart. Furthermore, virtually no carbonization was observed in each of theexperiments. Such superior results were not achieved in tests with anyother existing laser source which could be delivered via a non-toxic,durable, flexible fiber optic to a vein or artery within the body.

In view of the above description, it is likely that modifications andimprovements may occur to those skilled in the art within the scope ofthis invention. Thus, the above description is intended to be exemplaryonly, the scope of the invention being described by the following claimsand their equivalents.

What is claimed is:
 1. A system for the removal of atheroscleroticplaque comprising:a Holmium-doped laser energy source having a pulsedoutput in a wavelength range of about 1.9 to about 2.1 micrometers; andan optical fiber for conducting laser energy from a proximal end of saidoptical fiber to a surgical site at a distal end of said optical fiberto a surgical site at a distal end of said optical fiber, the proximalend of said optical fiber being optically coupled to the output of saidlaser energy source, said laser energy source providing at least about0.6 joules per square millimeter per pulse to a surgical site at thedistal end of said optical fiber for removal of atherosclerotic plaquesubstantially by vaporization.
 2. A system for the removal ofatherosclerotic plaque comprising:a laser energy source having a pulsedoutput and a wavelength range of about 1.4-2.2 micrometers; and anoptical fiber for conducting laser energy from a proximal end of saidoptical fiber to a surgical site at a distal end of said optical fiber,the proximal end of said optical fiber being optically coupled to theoutput of said laser energy source, said laser energy source providingsufficient energy at the distal end of said optical fiber to removeatherosclerotic plaque substantially by vaporization.
 3. A system forthe removal of atherosclerotic plaque comprising:a laser energy sourceincluding means for operating said laser energy source in a pulsed modewith an output wavelength in the range of from about 1.4 to about 2.2micrometers and for operating said laser energy source at an energysufficient to deliver energy to a surgical site of at least about9.5×10⁻³ joules per square millimeter per pulse for removal ofatherosclerotic plaque at the surgical site substantially byvaporization; an optical fiber for conducting laser energy from saidlaser energy source from a proximal end of said fiber to a surgical siteat a distal end of said optical fiber; and means for coupling an outputof said laser source to the proximal end of said optical fiber.
 4. Asystem as recited in claim 3 wherein said laser energy source comprisesa Holmium-doped laser with an output wavelength in the range of fromabout 1.9 to about 2.1 micrometers.
 5. A system as recited in claim 3wherein said laser energy source is a Holmium-dopedYttrium-Scandium-Gadolinium-Garnet laser having an output wavelength of2.088 micrometers.
 6. A system as recited in claim 3 wherein said lasersource comprises a Holmium-doped Yttrium-Aluminium-Garnet laser.
 7. Asystem as recited in claim 3 wherein said laser source comprises aHolmium-doped Yttrium-Lithium Fluoride laser.
 8. A system as recited inclaim 3 wherein said laser source comprises an Erbium-dopedYttrium-Lithium-Fluoride laser.
 9. A system as recited in claim 3wherein said laser source comprises a Thulium-dopedYttrium-Aluminium-Garnet laser.
 10. A system as recited in claim 3further comprising an aiming laser source generating a visible lightoutput and means for directing said visible light outoput through saidlaser source and said optical fiber to align said laser and said fiberto visually illuminate said surgical site.
 11. A system as recited inclaim 3 wherein said optical fiber further comprises:a plurality ofoptical fibers having proximal and distal ends; a plurality of partiallyreflective mirrors arranged in series along an axis of an output beamemitted by said laser source for directing a portion of said output beamof said laser source to said proximal ends of said optical fibers; and aplurality of focusing lenses positioned between said mirrors and saidproximal ends of said fibers for focusing portions of said laser outputto said proximal ends of said fibers; said fibers being held in a fixedposition relative to one another so that optical beams emanating fromsaid distal ends of said fibers overlap.
 12. A system in accordance withclaim 3 wherein the surgical site is accessible percutaneously, saidsystem further comprising means for percutaneously placing the distalend of said optical fiber.
 13. A system as recited in claim 3 whereinthe pulse width of each pulse is substantially less than about 83milliseconds.
 14. A system as recited in claim 13 wherein the pulsewidth is of the order of about 8 milliseconds.
 15. A system as recitedin claim 3, wherein said optical fiber comprises a silica furtherpurified to reduce the hydroxyl ion content sufficiently to allowtransmission of laser energy from said laser energy source to thesurgical site at said wavelength range of about 1.4 to about 2.2micrometers and at said energy of at least about 9.5 ×10⁻³ joules persquare millimeter per pulse.
 16. A method for the percutaneous removalof atherosclerotic plaque comprising the steps of:percutaneouslyinserting a catheter containing an optical fiber into the body; guidinga distal end of the optical fiber to a surgical site within the body;operating a laser energy source in a pulsed mode with an output beamhaving a wavelength in the range of from about 1.4-2.2 micrometers andan energy sufficient to deliver energy to a surgical site of at leastabout 9.5×10⁻³ joules per square millimeter per pulse for removal ofatherosclerotic plaque at the surgical site substantially byvaporization; directing the output of the laser energy source to aproximal end of the optical fiber; and directing laser energypropagating down the optical fiber to the surgical site at the distalend of the optical fiber.
 17. A method for the percutaneous removal ofatherosclerotic plaque in accordance with claim 16 wherein saidoperating step comprises the step of operating the laser energy sourceat a repetition rate in the range of from about 1 to about 10 pulses persecond.
 18. A method for the percutaneous removal of atheroscleroticplaque in accordance with claim 16 wherein said operating step comprisesthe step of operating the laser energy source at an output wavelength ofabout 1.90 to about 2.10 micrometers.
 19. A method for the percutaneousremoval of atherosclerotic plaque in accordance with claim 16 whereinsaid operating step comprises the step of operating the laser energysource with a pulse width substantially less than about 83 milliseconds.20. A method for the percutaneous removal of atherosclerotic plaque inaccordance with claim 16 further comprising the step of reducing thehydroxyl ion content of the optical fiber sufficiently to allowtransmission of laser energy from the laser energy source to thesurgical site at said wavelength range of from about 1.4-2.2 micrometersand at said energy of at least about 9.5×10⁻³ joules per squaremillimeter per pulse.
 21. A system for the removal of atheroscleroticplaque comprising:a laser energy source having a pulsed output in awavelength range of about 1.4-2.2 micrometers; and an optical fiber forconducting laser energy from a proximal end of said optical fiber to asurgical site at a distal end of said optical fiber, the proximal end ofsaid optical fiber being optically coupled to the output of said laserenergy source, said laser energy source providing at least about9.5×10⁻³ joules per square millimeter per pulse to a surgical site atthe distal end of said optical fiber for removal of atheroscleroticplaque substantially by vaporization.
 22. A system for the removal ofatherosclerotic plaque comprising:an Erbium-doped YAG laser energysource having a pulsed output at a wavelength of about 1.55 micrometers;and an optical fiber for conducting laser energy from a proximal end ofsaid optical fiber to a surgical site at a distal end of said opticalfiber, the proximal end of said optical fiber being optically coupled tothe output of said laser energy source, said laser energy sourceproviding at least about 9.5×10⁻³ joules per square millimeter per pulseto a surgical site at the distal end of said optical fiber for removalof atherosclerotic plaque substantially by vaporization.
 23. A systemfor the removal of atherosclerotic plaque comprising:an Erbium-doped YLFlaser energy source having a pulsed output at a wavelength of about 1.73micrometers; and an optical fiber for conducting laser energy from aproximal end of said optical fiber to a surgical site at a distal end ofsaid optical fiber, the proximal end of said optical fiber beingoptically coupled to the output of said laser energy source, said laserenergy source providing at least about 5.7 joules per square millimeterper pulse to a surgical site at the distal end of said optical fiber forremoval of atherosclerotic plaque substantially by vaporization.
 24. Asystem for the removal of atherosclerotic plaque comprising:aThulium-doped YAG laser energy source having a pulsed output at awavelength of about 1.88 micrometers; and an optical fiber forconducting laser energy from a proximal end of said optical fiber to asurgical site at a distal end of said optical fiber, the proximal end ofsaid optical fiber being optically coupled to the output of said laserenergy source, said laser energy source providing at least about 0.76joules per square millimeter per pulse to a surgical site at the distalend of said optical fiber for removal of atherosclerotic plaquesubstantially by vaporization.
 25. A system for the removal ofatherosclerotic plaque comprising:a Holmium-doped laser energy sourceincluding means for operating said laser energy source in a pulsed modewith an output wavelength in the range of from about 1.90 to about 2.1micrometers, for operating said laser energy source at an energysufficient to deliver energy to a surgical site of at least about 0.6joules per square millimeter per pulse, for operating said laser energysource with a pulse width substantially less than about 83 millisecondsand for operating said laser energy source at a repetition rate in therange of from about 1 to about 10 pulses per second; an optical fiberfor conducting laser energy from said laser energy source from aproximal end of said fiber to a surgical site at a distal end of saidoptical fiber; and means for directing an output of said laser source toa proximal end of said optical fiber.
 26. A system in accordance withclaim 25 wherein the surgical site is accessible percutaneously, saidsystem further comprising means for percutaneously placing the distalend of said optical fiber.
 27. A system for the removal ofatherosclerotic plaque comprising:an Erbium-doped YAG laser energysource including means for operating said laser energy source in apulsed mode with an output wavelength of about 1.55 micrometers, foroperating said laser energy source at an energy sufficient to deliverenergy to a surgical site of at least about 9.5×10⁻³ joules per squaremillimeter per pulse, for operating said laser energy source with apulse width substantially less than about 83 milliseconds and foroperating said laser energy source at a repetition rate in the range offrom about 1 to about 10 pulses per second; an optical fiber forconducting laser energy from said laser energy source from a proximalend of said fiber to a surgical site at a distal end of said opticalfiber; and means for directing an output of said laser source to aproximal end of said optical fiber.
 28. A system in accordance withclaim 27 wherein the surgical site is accessible percutaneously, saidsystem further comprising means for percutaneously placing the distalend of said optical fiber.
 29. A system for the removal ofatherosclerotic plaque comprising:an Erbium-doped YLF laser energysource including means for operating said laser energy source in apulsed mode with an output wavelength of about 1.73 micrometers, foroperating said laser energy source at an energy sufficient to deliverenergy to a surgical site of at least about 5.7 joules per squaremillimeter per pulse, for operating said laser energy source with apulse width substantially less than about 83 milliseconds and foroperating said laser energy source at a repetition rate in the range offrom about 1 to about 10 pulses per second; an optical fiber forconducting laser energy from said laser energy source from a proximalend of said fiber to a surgical site at a distal end of said opticalfiber; and means for directing an output of said laser source to aproximal end of said optical fiber.
 30. A system in accordance withclaim 29 wherein the surgical site is accessible percutaneously, saidsystem further comprising means for percutaneously placing the distalend of said optical fiber.
 31. A system for the removal ofatherosclerotic plaque comprising:a Thulium-doped YAG laser energysource including means for operating said laser energy source in apulsed mode with an output wavelength of about 1.88 micrometers, foroperating said laser energy source at an energy sufficient to deliverenergy to a surgical site of at least about 0.76 joules per squaremillimeter per pulse, for operating said laser energy source with apulse width substantially less than about 83 milliseconds and foroperating said laser energy source at a repetition rate in a range offrom about 1 to about 10 pulses per second; an optical fiber forconducting laser energy from said laser energy source from a proximalend of said fiber to a surgical site at a distal end of said opticalfiber; and means for directing an output of said laser source to aproximal end of said optical fiber.
 32. A system in accordance withclaim 31 wherein the surgical site is accessible percutaneously, saidsystem further comprising means for percutaneously placing the distalend of said optical fiber.
 33. A method for the percutaneous removal ofatherosclerotic plaque comprising the steps of:percutaneously insertinga catheter containing an optical fiber into the body; guiding a distalend of the optical fiber to a surgical site within the body; operating alaser energy source in a pulsed mode with an output beam having awavelength of about 1.55 micrometers and an energy sufficient to deliverenergy to a surgical site of at least about 9.5×10⁻³ joules per squaremillimeter per pulse, said laser energy source being operated with apulse width substantially less than about 83 milliseconds and at arepetition rate in the range of from about 1 to about 10 pulses persecond; directing the output of the laser energy source to a proximalend of the optical fiber; and directing laser energy propagating downthe optical fiber to the surgical site at the distal end of the opticalfiber.
 34. A method for the percutaneous removal of atheroscleroticplaque comprising the steps of:percutaneously inserting a cathetercontaining an optical fiber into the body; guiding a distal end of theoptical fiber to a surgical site within the body; operating a laserenergy source in a pulsed mode with an output beam having a wavelengthof about 1.73 micrometers and an energy sufficient to deliver energy toa surgical site of at least about 5.7 joules per square millimeter perpulse, said energy source being operated with a pulse widthsubstantially less than about 83 milliseconds and at a repetition ratein the range of from about 1 to about 10 pulses per second; directingthe output of the laser energy source to a proximal end of the opticalfiber; and directing laser energy propagating down the optical fiber tothe surgical site at the distal end of the optical fiber.
 35. A methodfor the percutaneous removal of atherosclerotic plaque comprising thesteps of:percutaneously inserting a catheter containing an optical fiberinto the body; guiding a distal end of the optical fiber to a surgicalsite within the body; operating a laser energy source in a pulsed modewith an output beam having a wavelength of about 1.88 micrometers and anenergy sufficient to deliver energy to a surgical site of at least about0.76 joules per square millimeter per pulse, said laser energy sourcebeing operated with a pulse width substantially less than about 83milliseconds and at a repetition rate in the range of from about 1 toabout 10 pulses per second; directing the output of the laser energysource to a proximal end of the optical fiber; and directing laserenergy propagating down the optical fiber to the surgical site at thedistal end of the optical fiber.